The technical part of the profession of an optician consists in mounting a pair of correcting ophthalmic lenses in an eyeglass frame selected by a wearer.
This mounting comprises at least three main operations:                acquiring the shape of a longitudinal profile representative of the shape of the outline of one of the surrounds of the selected eyeglass frame;        centering the ophthalmic lens under consideration, i.e. positioning and orienting said longitudinal profile appropriately on the lens, in such a manner that once machined to have this profile and then mounted in its frame, the lens is positioned correctly relative to the corresponding eye of the wearer, so as to enable it to perform as well as possible the optical function for which it is designed; and then        shaping the lens, i.e. machining its outline to have the shape of the longitudinal profile.        
With rimmed eyeglass frames, the surround (or “rim”) is designed to surround the entire periphery of the lens. Shaping then consists in a beveling operation which consists in forming an engagement ridge all along the edge face of the lens for the purpose of engaging in a groove, commonly known as a bezel, that runs around the inside face of the rim of the eyeglass frame.
With half-rimmed eyeglass frames, the surround comprises a half-rim that matches the top portion of the outline of the lens, and a string that runs along the bottom portion of the outline of the lens in order to hold the lens against the half-rim. Shaping then includes a grooving operation that consists in forming an engagement groove along the edge face of the lens, with the top portion of the groove serving to receive a ridge provided along the bottom face of the half-rim and with the bottom portion of the groove serving to receive the string.
Generally, in order to ensure that the engagement ridge or groove does not overflow into the front or the rear of the edge face of the lens, the optician machines the lens in such a manner that the engagement ridge or groove follows the front optical face of the lens, so that it extends at a constant optical distance from said front optical face.
It is also known to machine the lens in such a manner that the engagement ridge or groove extends halfway across its edge face.
Nevertheless, neither of those two methods is flexible.
It can thus happen that once a pair of eyeglasses has been assembled, its appearance is esthetically unpleasing because of the way the lenses are positioned in the surrounds of the frame in compliance with the above-mentioned constraint. In particular, it is sometimes found that if the edge face of the lens is particularly thick, then it projects in unsightly manner from the rear of the surround.
It is also sometimes found to be impossible to mount the lens as a result of interference between the rear edge of the edge face of the lens and the corresponding nose pad of the eyeglass frame.
The likelihood of this occurring is made worse in that eyeglass frames exist in a very wide variety of shapes, as do ophthalmic lenses.